Remove the roses from their vase. Re-cut the stems and submerge the whole rose – stem, leaves, flowers and all – in a sink or tub of warm water. Leave the roses submerged for 30 minutes. Use that time to clean and refill the vase with fresh water and a bit of floral preservative.
Re- cut the stems on a slant, underwater if possible and place the roses back in the vase. You will have perky fresh roses to enjoy for a week or more.

Re-cut the stem bottoms and submerge them in very warm (not so hot you can't touch it) water and let them sit for about and hour before replacing them in the vase - This should ope their vasular system and let the water rise through the stem.